Computed tomographic colonography (CTC), or virtual colonoscopy (VC), has been confirmed as a viable and cost-effective alternative to optical colonoscopy for colon cancer screening by large-scale and multi-center clinical trials. Cathartic bowel preparation is one of the major drawbacks for the wide acceptance of CTC as a mass screening tool. The main technical barrier to applying non-cathartic bowel preparation to CTC examination (ncCTC) is the lack of an accurate visualization method to subtraction of tagged fecal residuals in CTC images. Current solution is applying an image processing method named electronic cleansing (EC), or digital bowel cleansing, for removing tagged fecal regions in CTC images after image acquisition. However, an accurate and artifact-free EC solution has not been found in clinical research, in particularly for ncCTC. Applying dual-energy CT (DE-CT) to ncCTC image acquisition (nc-DE-CTC) provides a promising solution to differentiating tagged fecal materials from colonic soft-tissue structures. However, current EC methods in DE-CTC utilize only the differences in Hounsfield (HU) value to characterize tagged fecal materials. These methods are thus heuristic and incomplete to overcome the major EC artifacts in nc-DE-CTC. Based on our recent finding that the DE-CT derived effective atomic numbers (Zeff) by fractions of atomic mass number are substantially higher in air and iodine-tagged fecal materials (the two substitutes in colonic lumen) than those in soft tissue and fat, we will develop an innovative dual-energy EC (DE-EC) method, named ZEC, by utilization of a Zeff transfer function for direct visualization of the colonic soft-tissue structures and suppression of tagged fecal materials without explicit segmentation and removal of tagged fecal materials in CTC images. The central hypothesis is that the DE-CT derived effective atomic number (Zeff) is an accurate physical parameter for characterizing colonic lumen (air and the tagged fecal residuals) from colonic soft-tissue structures in nc-DE-CTC images, which provides the physical model for the development of the proposed ZEC method. The primary aim is to develop a Zeff-based DE-EC scheme for direct visualization of nc-DE-CTC images without applying explicit segmentation of tagged fecal regions in conventional EC, and to evaluate the performance of the proposed ZEC scheme in reading of nc-DE-CTC in reference to the ?ground truth? established by optical colonoscopy. The successful development and validation of the proposed ZEC scheme will deliver an accurate and efficient solution for artifact-free visualization of ncCTC images, substantially improve patient adherence and compliance to undergoing CTC examination, and thus promote the acceptance of CTC as a highly reliable and cost-effective colon cancer screening strategy.